Are you talking about Rremote Access VPN to the MX? I am not aware of any config to restrict the VPN-clients IP.
Not sure if it is worth the effort, but if you authenticate the VPN-user with RADIUS, you could filter on the RADIUS-Attribute "Calling-Station-ID" which is the IP of the remote client.
EDIT: I just remembered (and quickly confirmed that this is also the case with MX version 15.37) that the MX does not send this RADIUS attribute. So this "workaround" will not work as it would for example with a Cisco ASA/FTD!
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